USB: cdc-acm: fix pipe type of write endpoint

If the write endpoint is interrupt type, usb_sndintpipe() should
be passed to usb_fill_int_urb() instead of usb_sndbulkpipe().

Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>